Anode and method of production thereof

ABSTRACT

An electrode useful for electrowinning and other processes including evolution of oxygen or chlorine or for plating is formed of titanium particles, compacted cold, and coated and cemented with a first layer of manganese dioxide thermally deposited from Mn (NO3)2 on the grains to form a coating of a combination of manganese dioxide and titanium oxide having a rutile crystal structure, and a further outer layer of manganese dioxide which is electrodeposited, the two coatings not being limited to the surface of the electrode but extending to all exposed surfaces of the grains including those which are walls of channels between the grains. The compression of the titanium powder is to a density between 30 and 70 percent of the density of solid metal. Solid metal such as metal mesh may be included in the titanium powder prior to compacting, such as expanded titanium metal, to assist in strengthening the form. In an alternate embodiment the metal substrate is lead.

United States Patent Feige, Jr.

ANODE AND METHOD OF PRODUCTION THEREOF Inventor: Norman C. Feige, Jun,Ridgefield Ave., R.F.D. 1, South Salem, N.Y. 10590 Filed: May 3, 1974Appl. No.: 466,752

Related U.S. Applimtinn Data Division of Ser. No. 380,325, July 18,1973, Pat. No. 3,855,084.

U.S. Cl 204/286; 136/120 R; 136/138; 204/83; 204/284 Int. CL. C251!1/00; C2513 11/02; C25C 7/02; HOlM l/00 Field of Search 204/284, 290 R,290 F, 204/83, 286; 136/138, 120 R References Cited FOREIGN PATENTS ORAPPLICATIONS 7/1971 Germany 204/290 F Primary Examiner-F. C. EdmundsonAttorney, Agent, or Firm-Wheeler, Morsell, House & Fuller [57] ABSTRACTAn electrode useful for electrowinning and other processes includingevolution of oxygen or chlorine or for plating is formed of titaniumparticles, compacted cold, and coated and cemented with a first layer ofmanganese dioxide thermally deposited from Mn (N09 on the grains to forma coating of a combination of manganese dioxide and titanium oxidehaving a rutile crystal structure, and a further outer layer ofmanganese dioxide which is electrodeposited, the two coatings not beinglimited to the surface of the electrode but extending to all exposedsurfaces of the grains including those which are walls of channelsbetween the grains. The compression of the titanium powder is to adensity between 30 and 70 percent of the density of solid metal. Solidmetal such as metal mesh may be included in the titanium powder prior tocompacting, such as expanded titanium metal, to assist in strengtheningthe form. In an alternate embodiment the metal substrate is lead.

4 Claims, 4 Drawing Figures ANODE AND METHOD OF PRODUCTION THEREOF Thisapplication is a division of application Ser. No. 380,325, filed July18, 1973, now U.S. Pat. No. 3,855,084.

BACKGROUND OF THE INVENTION It has long been known that titanium metalhas superior properties for use as an electrode in cells, baths orsolutions which would corrode and be contaminated by metal fromelectrodes of other compositions. Nevertheless there are difficulties inthe use of a titanium electrode as well. Solid titanium leads to arequirement for high voltage, and gives poor current efficiency. FoxU.S. Pat. Nos. 2,631,115 and 2,608,531 discuss some of the difficultiesin connection with specific systems, for instance the passive surfacecoating on the titanium, and indicate that at least in the production ofoxides such as manganese dioxide for use in batteries as a depolarizer,the use of a porous titanium anode made of chips is of assistance. Undercertain special conditions described by Fox, the plating of MnO or in abattery, the use of a specific form of titanium chips of about 35 meshin one case and the use of specific voltage relationships duringelectrolytic formation of the surface of the electrode prior to use inthe other patent, improved results are observed using porous titaniumanodes. Fox discloses coating the surfaces of the porous titanium masswith graphite, gold or iron, or with any good conductor which is inertto the electrolyte in which the electrode is to be used. Among otherthings Fox discloses that his electrodes are useful for the preparationof electrolytic manganese dioxide. His described electrode does notfunction as an oxygen evolution anode, but passivates under thoseconditions.

In industry it is most important to select a suitable anode that .doesnot contaminate the electrolyte or contaminate the cathode deposit, thathas a long life, and has a low oxygen overvoltage during electrolysis.Platinum is an excellent known anode material which satisifies the abovementioned characteristics.

Recently platinum and other precious metals have been applied to atitanium substrate to retain their attractive electrical characteristicsand further reduce manufacturing cost. However such anodes are expensiveand are not suitable for some industrial uses. Thus carbon and leadalloy electrodes have generally been used. The carbon anode, however,has the disadvantage that it greatly contaminates the electrolyte, wearsfast, and has high electrical resistance which results in an increase incell voltage. It may also be degraded to CO during oxygen evolution. Thedisadvantage of the lead alloy anode is that PbO- changes to Pb ,0 whichis poorly conductive. gets below this layer and flakes off the film.These particles become trapped in the deposited copper at the cathode,degrading it.

In order to overcome these disadvantages it has recently been proposedto plate the surface of a titanium substrate with platinum and toelectrodeposit either lead dioxide or manganese dioxide on the platedsurface. Such anodes have the disadvantage of comparatively high oxygenovervoltage. In addition both coatings have high internal stress whenelectrolytically deposited and are liable to be detached from thesurface during commerical usage, both contaminating the electrolyte and,in the case of lead, being deposited on the cathode to reduce its value.Thus current density with such anodes is very limited.

To improve the high oxygen overvoltage it has also been proposed tocompact and sinter titanium chips to increase the apparent surface area.Such an anode has somewhat improved characteristics but does notproperly receive and retain the electrodeposited manganese dioxidecoating.

This invention is based on recognition that deposited manganese dioxideis both insoluble and electrically conductive, and cannot readily bedeposited as a reduced product on the cathode. Experiments have shownthat my electrode has a low oxygen overvoltage during electrolysis,economizes on electric power necessary for electrolysis, reduces theloss of manganese from the electrode to the bath to a very low figure,and is believed to be the optimum electrode for electrolytic winning ofcopper, zinc and nickel in sulfate electrolytes. It is also useful forevolving oxygen in sulfate systems and chlorine in chloride systems.

SUMMARY OF THE INVENTION The preferred electrode of my invention is madeof titanium powder cold compacted to the shape of an electrode, thecompaction being sufficient to produce a density in the powder between30 and percent of the density of the solid metal of which the powder iscomposed. A layer of manganese dioxide is produced on the surface of thegrains throughout the mass of the electrode by thermal decomposition ofMn (NO That layer is modified by ion exchange with the titanium dioxidesurface layer and probably with the titanium metal of the powder grainsto contain titanium atoms as well as manganese atoms. These do notsignificantly modify the crystal structure which is essentially thatofrutile. This mixed structure is formed while the material of thecoating is in the intermediate manganeous-manganite form during thedecomposition from manganeous nitrate to manganese dioxide.

A final coating of manganese dioxide is electrodeposited over the hybridcoating.

Preferably the structure is strengthened by including solid metal in theelectrode. Apreferred form is an expanded metal lattice of titaniumalthough other shapes may be used. The preferred size range of thetitanium powder is mesh to -325 mesh. The preferred thickness of theouter coating of maganese dioxide is 100 microns. The density referredto is calculated on the weight of the powder alone. The upper limit isthe loss of connected pores between the particles.

The preparation of an electrode in this manner produces a strongerelectrode without the necessity of sintering the powder because theinitial thermally deposited layer is extremely effective in cementingthe grains of the powder. It has been found that even loose titaniumpowder may be cemented by this method to produce a coherent shape.Powder which has been cold compacted to between 30 and 70 percent ofmetallic density and then coated as described produces an extremelydurable electrode suitable for use as the anode in an electrowinningprocess. The anode evolves O, in sulfates and C1 in chlorides, andaccordingly may also be used for the evolution of chlorine as well.

It has been observed that manganese dioxide coating of prior artelectrodes used in electrowinning is lost during periods of shutdown, atwhich time it goes into solution and becomes a part of the bath,degrading the electrode. The loss is largely in the form of conversionto manganese cations and permanganate. With the anode of my invention,the ions and permanganate are largely limited to the pores or channelsbetween the grains of titanium powder and a very high percentage isredeposited as manganese dioxide upon application of current for theelectrowinning process, in the same manner that the outer layer wasoriginally deposited. Thus degradation of the anode and pollution of thebath are both avoided.

Finally the electrical and operating characteristics of my electrodecompare favorably with prior art electrodes.

The chief presently known use of my electrode is as an anode forelectrolysis. So used, an advantage of the anode is low oxygenovervoltage during electrolysis, thus economizing the electrical powernecessary.

A simlar electrode may be made with lead particles, at lower cost. As inthe case of the titanium electrode a first coating of MnO is thermallydeposited and forms a hybrid with the lead oxide surface layer normallypresent. A further layer of MnO is electrodeposited. A reinforcement ofsolid metal, which may be titanium expanded metal mesh, may be used.

DRAWINGS FIG. 1 is a perspective view of a rectangular electrode madeaccording to my invention with portions broken away to show interiorstructure.

FIG. 2 is an enlarged cross-sectional view on line 22 of FIG. 1.

FIG. 3 is a graph of cell voltage against current density for an anodeaccording to my invention compared with a lead anode and a solidtitanium anode coated with MnO FIG. 4 is an idealized cross-section ofseveral grains showing the two layers and the pores highly magnified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the disclosure hereinis detailed in order to enable those skilled in the art to practice theinvention, the embodiments disclosed merely exemplify the inventionwhich may take other forms. The scope of the invention is defined in theclaims.

FIG. 1 is a broken away perspective view showing a portion of anelectrode 10 formed according to my invention. An expanded metal mesh 12of titanium metal has been placed with l mesh titanium powder in a moldand compacted in a press until the powder has a density of 30 to 70percent of theoretical density of the metal. The precise shape is not apart of my invention and may conform to specifications for a particularuse, so the outline is not shown. FIG. 2 is a cross-sectional view. Asshown in FIG. 4, which is a highly magnified and idealizedcross-sectional view through several particles of titanium, each grain20 is coated with a first layer 22 and a second layer 24, leavinginterconnected pores 26 between the grains.

The first layer 22 is basically MnO which has been deposited by thermalbreakdown of Mn (NO and which has exchanged ions with the metal andsurface layer of titanium oxide normally present on the metal and notseparately shown. Thus layer 22 has a modified rutile character. It isstrongly adherent and cements the grains 20 and the mesh 12 together.

The second layer 24 is electrodeposited MnO It is not known tointerchange ions with the first layer and is believed therefore to beidentical with commercially electrodeposited manganese dioxide.

The pores 26 are the spaces between the coated grains. As more fullydescribed elsewhere, they are highly interconnected. This not onlycreates high available surface area (as opposed to surface in a closedcavity not connected through pores 26 with the exterior of the electrode10) thus improving the apparent current density, it also means that asubstantial part of the first and second layers 22-24 are in the pores.During inactivity of the immersed electrode Mn0 can break down tomanganese cations and permanganate. In my electrode these are largelytrapped in pores 26. Upon reapplication of current to the anode, MnO isre-formed on the second layer with very little loss.

The method of my invention thus comprises the basic steps of compactingmetal particles to a density sufficient to make a coherent final productbut not so high as to destroy the high interconnection between thepores, thermally breaking down Mn (NO at a temperature between about C.and 475 C. to form a first layer on the particles or grains, anddepositing a second layer of MnO over the first layer, to form anelectrode. The metal may preferably be titanium but may also be lead.

The density of the compacted titanium powder used in my electrode hasboth a lower and upper limits for satisfactory performance. The freelypoured titanium powder of a typical sieve analysis as follows:

TABLE I Ti Powder Analysis Typical Sieve Analysis Size All powder lessthan l00 mesh.

Typical screen analysis:

-l00. mesh 25 percent l50. +200 mesh 21 percent 200. +270 mesh 26percent 270, +325 mesh 17 percent 325 mesh 11 percent Chemistry Oxygen0.30 max. Nitrogen 0.04 max. Carbon 0.04 max. Chlorine 0.20 max.

Iron 0.50 max. Magnesium 0.30 max.

Total others 0.12 max. Titanium 98.50 min.

has a theoretical density of 25 percent, that is, its weight is 25percent of the weight of an equal volume of solid titanium metal havingthe same analysis. Using great care it is possible to compact suchpowder to only 30 percent density in the shape of an anode and cementthe grains of the powder together by thermal decomposition of Mn (NO asdescribed elsewhere in this specification. Thus 30 percent density isbelieved to be a practical lower limit of density. With greatercompacting force the density of the powder may be increased to about 70percent while retaining highly interconnected passageways between thegrains of metal of sufficient size for the application of the coatingsof my electrode. Above 70 percent theoretical density the pores loseinterconnection and surface area is lost to the extent that followingthe process described in this specification does not result indeposition of sufficient MnO either in the layer produced by thermaldecomposition or in the electrodepositedlayer. Above 70 percent densitythe oxygen overvoltage .rises unduly as shown in table 2 below:

TABLE 2 p The Relationship Between Theoretical Density And AnodePerformance Another method of defining the upper limit is by blowing airthrough the compacted mass. Substantial resistance to the passage of airindicates that few passages are interconnected, showing that compactionis too great. The upper limit may vary with particle size and shape butincludes only a degree of compaction which leaves the poreshighlyinterconnected.

In my electrode, the grains of titanium powder are cemented together bythe first coating of MnO which is produced by thermal decomposition ofMn (N between 120 C. and 475 C. During the thermal decomposition thereis an extensive reaction with the surface titanium oxide layer which isnormally present on titanium metal by ion exchange, during the phasewhen the coating is black liquid manganeous-manganite. The result is anadherent coating bonding the grains of titanium powder with a highlyconductive oxide film. Subsequently a further layer of MnO, iselectrolytically deposited, resulting in an electrode having extensivelyinterconnected passages between the grains, the surface of each grainboth on the surface of theelectrode and in the passages being coatedwith'a first layer of modified rutile containing titanium, and then withMnO Electrodeposited manganese dioxide is brittle, and

- has large internal stresses. It is readily detached from a substratewhen deposited to an appropriate thickness for electrode use, making itdifficult to form an effective and long lived electrode from suchmanganese dioxide alone. My invention'permits the application of heavydeposits in excess of 100 microns thick of manganese dioxide within thepores between the grains of titanium powder, thus taking advantage ofthe large internal stresses of the coating to improve its adherencerather than to cause failure as in the case of a flat sheet electrode.

Finally, by depositing much of the coating internally it is protectedboth from mechanical dislodgement and from loss into the solution whenthe electrode is inactive.

Although under anodic potential as applied during electrolysis,manganese dioxide is insoluble, under an open circuit permanganate isobserved in solution. As the potential is again applied to the anode thepermanganate is redeposited on the anode as manganese dioxide. Bycoating the porous substrate internally the dissolution of manganesedioxide is reduced and the concentration of permanganate is sufficientlyhigh in the pores for redeposition to reduce significantly the loss ofmanganese from the anode and the pollution of the bath.

EXAMPLE 1 An example showing the comparison between my electrode and asimilar electrode made with solid titanium plate is as follows:

Two sheets of expanded titanium mesh, each 1.5 millimeters thick, wereplaced in a mold 500 centimeters by 7.50 centimeters by 3 millimetersthick with 82 grams of titanium powder having the sieve analysis andchemical analysis shown in Table 1. These were subjected to a pressureof 5,400 kilograms, resulting in a composite structure in which thepowder component had a theoretical density of 52 percent of the densityof solid titanium. The electrode was impregnated with an aqueoussolution of Mn (N0 and was baked at 176 Centigrade. The electrodedeveloped a gray adherent coating. The electrode was then placed in abath of MnSO, and H 80, at Centigrade and current was applied toelectrolytically deposit a coating of manganese dioxide followingaccepted techniques. It was found that during this step current densitycould be varied from 0.1 ampere to 6.7 amperes per square foot, whichwas the observed oxygen evolution value for the anode. A black layer ofMnO developed over the first layer.

In the same manner a titanium plate 500 centimeters by 750 centimetersby 3 millimeters thick having a sandblasted surface was coated with twolayers as described above.

The two electrodes thus produced are compared in FIG. 3 along with anelectrode of solid lead. The bath was a copper plating bath containingCuSO and H 80, at 60 Centigrade and a gap of 1% inch. The anode madefrom titanium plate was less durable and required higher cell voltage,despite the use of both thermal and electrolytic deposition of MnO inaccordance with a portion of my invention. Line 34 is the curve for theporous anode with two layers made according to my invention. Line 32 isthe curve for lead. Line 30 is the curve for the solid titanium anodewith two Mn0 layers.

A porous anode as described in this example was used for electrolyticwinning of copper, zinc and nickel in the respective commercialsulfate-sulfuric acid electrolytes. The anode performed satisfactorilyand exhibited a substantial improvement in cell voltage in the system ascompared to lead anodes in each electrolyte.

The porous electrode described has also been tested as an anode forevolution of oxygen, and as an anode for evolution of chlorine, bothwith good efficiencies and service life. The anode is useful forchlorination of water, for instance.

EXAMPLE 2 The effect of changes in the density of the powder componentof my electrode may be seen in table 2 be- 5 low. Anodes preparedaccording to the first paragraph of Example 1, with the exception thatthe compaction and amount of Mn'O varied, are compared.

TABLE 2 The Relationship Between Theoretical Density And AnodePerformance In a manner like that of Example 1, lead electrodes wereprepared using lead shot particles and titanium expanded metal mesh butusing lower pressure. lln respective trials lead particles ranging fromno. 6 shot (0.l 10 in.) to no. ll shot (0.065 in.) were used. After thetwo layers of MnO, are deposited as in Example I, the electrode wastested as an anode in a copper sulphate and sulfuric acid conventionalelectrolyte. The graph of cell voltage vs. amperes/sq. ft. for thiselectrode was intermediate between such a graph for a solid lead anodeand that for the anode of Example I! for the particle sizes tested.

It will be understood that the invention is not limited to the examplesdescribed and that many modifications may be introduced therein. Thescope of the invention is intended to be limited only by the scope ofthe appended claims.

I claim:

1. An electrode formed from compacted metal particles comprisingtitanium, the grains being covered with a first layer of Mno depositedby thermal decomposition of Mn (N09 the particles being further coveredwith an outer layer of manganese dioxide, an extensively interconnectedpore system being left between the compacted particles, the particlesbeing compacted along with at least one solid metal reinforcement toincrease the mechanical strength of the electrode.

2. The electrode of claim 1 in which the particles are cemented togetherby the first layer.

3. The electrode of claim 1 in which the particles are titanium havingsizes predominantly between 100 mesh and 325 mesh, the particles beingcompacted to between 30 and percent of the density of the solid metal ofwhich the particles are composed.

4. The electrode of claim 1 in which the particles are titanium with atitanium oxide surface film and in which the first layer contains atomsfrom the surface film and thereby comprises a mixture of titanium oxideand manganese dioxide.

1. AN ELECTRODE FORMED FROM COMPACTED METAL PARTICLES COMPRISINGTITANIUM, THE GRAINS BEING COVERED WITH A FIRST LAYER OF MNO2 DEPOSITEDBY THERMAL DECOMPOSITION OF MN (NO3)2 THE PARTICLES BEING FURTHERCOVERED WITH AN OUTER LAYER OF MAGANESE DIOXIDE, AN EXTENSIVELYINTERCONNECTED PORE SYSTEM BEING BETWEEN THE COMPACTED PARTICLES, THEPARTICLES BEING COMPACTED ALONG WITH AT LEAST ONE SOLID METALREINFORCEMENT TO INCREASE THE MECHANICAL STRENGTH OF THWE ELECTRODE. 2.The electrode of claim 1 in which the particles are cemented together bythe first layer.
 3. The electrode of claim 1 in which the particles aretitanium having sizes predominantly between 100 mesh and 325 mesh, theparticles being compacted to between 30 and 70 percent of the density ofthe solid metal of which the particles are composed.
 4. The electrode ofclaim 1 in which the particles are titanium with a titanium oxidesurface film and in which the first layer contAins atoms from thesurface film and thereby comprises a mixture of titanium oxide andmanganese dioxide.